Marmot - Trestles 0

If you only plan to car camp, don't bother with the down sleeping bags because the main benefits of down (compressible, lightweight) aren't really important features for car camping. On the other hand, the main benefit of synthetic insulation is that it still works if you get it wet, and it is cheaper. I prefer synthetic insulation for car camping.  I had a Marmot Trestles 0 degree for years and used it when I was a trail worker camping 7 nights in a row and it never let me down. Big and heavy for carrying in a backpack, but supremely warm. Sleeping 2 people in a car, the 15 degree would probably be great. You can always bring some extra blankets to supplement the sleeping bags.  I do a lot of trips where I sleep in the back of my vehicle. I've also done a lot of backpacking trips where I carry all of my stuff and sleep on a combo lightweight air mattress and foam pad. Take this advice that I learned the hard way - skip the air mattress for car camping. Air does not provide insulation, and usually will wind up having weird gaps between the mattress and the vehicle. Sounds like you already have a mattress topper, but maybe there is one that's a better fit in your vehicle? Foam is way warmer than air.  Also, maybe consider a layer or two of a more dense eva foam to go under your mattress topper. Sold as anti-fatigue mats, and come as single pieces or 2 foot by 2 foot puzzle pieces. Put that down under your mattress topper and your bed will be comfy, warm, and un-poppable. We roll up our mattress topper & bedding during the day and leave the anti fatigue mat rolled out with our stuff on it. We have [two of these](https://www.homedepot.com/p/TrafficMaster-Diamond-Soft-Foot-Black-4-ft-x-6-ft-Indoor-Outdoor-Commercial-Door-Mat-6016909004x6/306932662). Trim to size as needed.  Another hot take - don't build a platform other than a basic low profile sheet if you need it. In my opinion they wind up adding weight and taking up all of the space inside and leave you sleeping in a coffin. For a smaller vehicle like a highlander you're better off having your stuff well packed and just moving some stuff to the front seats or outside while you sleep. Being able to sit up in the middle of the night is priceless to me so I'll take the 5 minutes of shuffling stuff to the front if it means I don't get claustrophobic at night.  I've had a lot of great nights out camping in my vehicle and I love it. I really hope this helps, and hope you can create a setup that works well and comes in under budget! 

Not sure if weight matters, I have an older version of the marmot trestles 0°f/ -18°c synthetic fill. I find it to be a very warm bag. I have been comfortable in that bag sleeping in my jeep on a cold Vermont night. It’s $200 usd not exactly the most packable or lightest bag I own but it’s def warm.

Love my Marmot Trestles!

Marmot Trestles line are good deals on good bags. If you’re younger, just get a blue foam pad.

For a lot of backpacking brands, if you buy 2 bags; one with a left zipper and one with a right zipper ; you can simply zip them together (along with using them independently. I have Marmot bags that do this so my wife and I can carry our own gear but sleep together.

I used to use a Sea to Summit Womens Etherlight Xtreme pad. It turns out it's just too cold for me. Even though it has been tested, I believe it's so thick that the air moves around when I toss and turn, churning up cold air. I tried a Neo Air Xlite. It's not comfortable for me, but incredibly warm. I settled on the Nemo Tensor All Season. I also have an Exped pad. While it's warm enough, it's not as comfortable as the Nemo. Make sure your pad is warm enough. If the cold is coming from the ground, it's the pad, not the bag that needs to be changed out. I found that I sleep about 20 degrees colder than most ratings. So, I use an Enlightened Equipment Conundrum 10 Degree for the colder weather. Then go to a 20-degree Enlightened Equipment Revelation. The Conundrum has a 3/4 zip, which allows for the warm air to stay inside the bag. It does not unzip the way, so it won't lay flat. I sleep with a buff on my neck and a fleece hat. I'm warm at 34 degrees, but I also need to use my fleece or puffy. At 25 degrees, I start getting cold. I have also tried a Marmot bag and an REI bag. They weren't warm enough for me. I believe Katabatic and Featheted Friends may use more down, thus making their bags or quilts warmer. They are more expensive than EE. In April and May, I can switch to my Revelation, which does open all the way. I would not use the Conundrum in zero degree weather because I sleep cold. Nor would I ever choose to be out in zero degree weather. I have a 40-degree quilt that I made for summer use. I sleep cold enough that I am uncomfortably chilled in it when it's about 60 degrees at night. Some people will tell you the EE quilts are not warm enough. They used to use less down but have changed it to match the standards that most others use to achieve the loft needed for a designated temperature. You should be able to see the fill weight on each manufacturers website. Pay attention to the width and length of the quilts or bags. It will help you compare apples to apples. Only you will be able to tell what will be warm enough for you. I don't like using a mummy bag, so the quilts work well for me. You could consider using a liner. It may help boost your bag or quilts rating. I think you will get more warmth for the ounce by getting a warmer sleeping pad. You will probably need to practice in your backyard or some safe place to help you determine what works for you. It frequently test my gear in my backyard, so if it fails, I'm steps away from the house. You may also want to consider purchasing from an entity that allows returns, like REI or MEC. I also sleep with warm socks. I tried Alpaca wool, which worked well. However, I found that using two pairs of Alpha Direct fleece socks works just as well. I bought one a size bigger for layering. This allows me to bring two pairs when needed or leave one pair home in warmer months. You have to look at your sleep system as a system, not as individual components. Combing my puffy, fleece, hat, socks, and yes, even gloves, help me adjust to different temperatures.

I second the 2 sleeping bags instead of one, that way you can really bundle yourself up in there. I have a Marmot one and it’s excellent!